TestComplete12: Excel sheet not found error when executing as batch
I'm still thinking it's a permissions thing in some way... Running it at the "Run" prompt in windows is running it using your user account. Running it as a task in task scheduler or as a batch, potentially, may be using a different user account or privilege level or, if it's running as you, it MIGHT not be running with the most elevated privileges you have (admin).
Try in your task setting the security options to have it run as an administrator. I know it seems strange, but give it a shot and see if that works.